1. Field of the Invention
The present invention relates generally to a serial printer and, more particularly, to a serial printer including a synchronous signal generating circuit for synchronizing a movement of a carriage mounted with a printing head with a recording action of the recording head.
2. Related Background Art
A serial printer performs a record (print) while causing a carriage mounted with a printing head of a recording means to scan across a recording medium. If a carriage speed fluctuates due to some influence, however, a scatter in density appears as a result of recording. Especially in a color printer, the problem is a deviation in terms of color registration.
One of known methods of obviating these problems has hitherto involved steps of detecting a moving quantity of the carriage mounted with the recording means with respect to an apparatus body and performing a recording action through the recording means while synchronizing with this detected result.
More specifically, a scale portion of a linear encoder is fixed to the apparatus body. The carriage which moves relatively to this scale portion is mounted on a detecting portion of the linear encoder. On the other hand, an output signal from this detecting portion is amplified and thereafter fetched outside the carriage. A recording signal is generated in synchronization with this amplified signal, thereby preventing occurrences of the scatter in the printing density and of the deviation in the color registration.
An example of the prior art will be explained with reference to the drawings. FIG. 44 is a circuit diagram showing a configuration of a synchronous signal generating circuit in the conventional example. The scale portion of the linear encoder is mounted in the carriage and fixed to the apparatus body. A detecting portion 101 of the linear encoder detects a relative moving position of the carriage with respect to the apparatus body by detecting the scale portion. The detecting portion 101 consisting of MR elements which act based on a magnetic resistance effect is provided integrally with a pair of magnetic detecting elements 102, 103. This detecting portion 101 is also connected to a substrate 5 mounted on the carriage and shown by a broken line in the Figure. Connected, as known well, to this substrate 5 are amplifiers 104, 105 constituting constant current circuits, an amplifier 106 for amplifying a detected signal and a comparator 107. An output signal 303 is thereby outputted. Then, a variable resistor 158 for determining a reference voltage is connected to the comparator 107 and packaged on the substrate 5. An adjustment thereof is thus made on the carriage.
The operation of the thus constructed circuit will be explained. The magnetic detecting elements 102, 103 are supplied with a constant current via the constant current circuits 104, 105, respectively. Magnetic patterns are previously recorded at a fixed interval on the scale portion of the linear encoder which is fixed to the apparatus body. The detecting portion 101 moves along the scale portion. With this movement, resistance values of the magnetic detecting elements 102, 103 vary. The variation in the resistance value is detected as a change in voltage and amplified by the amplifier 106. An amplified signal is inputted to one input terminal of the comparator 107. This comparator 107 compares the amplified signal with a reference voltage preset by an adjustment of the variable resistor 158 and inputted to the other input terminal of the comparator 107. An output signal 303 is thereby obtained as a synchronous signal.
Further, an adverse effect may be exerted on the printing/recording result because of a high dependency on temperatures according to a detecting device and a circuit system. A detailed explanation will be given based on the drawings. FIG. 45A is a diagram showing a relationship of the reference voltage versus the signal inputted to the comparator 107. FIG. 45B is a pulse waveform diagram showing a relationship of the output signal 303 of the comparator 107 in combination with FIG. 45A. An input signal 301 to the comparator 107 takes, as depicted in the Figure, a waveform approximate to a sine waveform which varies with a fixed period.
On the other hand, in the pulse-shaped output signal 303 of the comparator, in consequence of obtaining the reference voltage as a threshold value, a difference between the input signal 301 and the reference voltage 302 appears, as can be understood from the Figure, in the form of a duty change in the output signal. If the recording/printing action is executed in synchronization with this output signal 303, the scatter in the density and a ruled-line deviation in an output image are caused. This results in a remarkable decline in terms of recording quality.
FIGS. 46A and 46B are explanatory views of the recording action, showing how dots D are recorded on a recording medium by driving a recording means in synchronization with the output signal 303 described above. As illustrated in the Figure, a fluctuation in pitch P between the dots D can be seen, and consequently, the scatter in the density is produced as a result of recording. Particularly in the color printer, this may cause the deviation in the color registration.
As explained above, in the conventional apparatus, the recording/printing action is effected synchronizing with the output signal. Therefore, the duty change in the output signal pulse waveform leads directly to the decline in quality as a result of printing.
Further, in the conventional apparatus, a means for restraining the duty change in the output signal pulse waveform depends on a stability of the circuit elements themselves. Hence, expensive parts have to be employed. A problem arises in terms of increasing the costs.
Additionally, when examining the conventional example from a different point of view, the temperature dependency is high according to the detecting device and the circuit system as well in the example of the prior art. The adverse influence may be therefore exerted on the printing/recording result. FIG. 47 is a graphic chart showing a temperature dependency characteristic with respect to a magnetic resistance effect rate of the MR element. FIG. 48 is a graphic chart showing a temperature dependency characteristic with respect to a resistance value of the MR element. An output of this MR element is expressed by the following formula: EQU V.sub.s =K.times.(.DELTA..rho./.rho.).times.R.times.i
where k is the constant, .DELTA..rho./.rho. is the magnetic resistance effect rate, R is the electric resistance, and i is the rated current.
The MR element expressed by the preceding formula and shown in FIGS. 47, 48 has the large temperature dependency characteristic, and hence, its output becomes as illustrated in FIG. 17.
The following is an explanation of actions in a case where such an MR element is employed in the detecting portion of the linear encoder. FIG. 49A is a waveform diagram showing relationship of the reference voltage 302 versus the signal 301 inputted to the comparator 107. FIG. 49B is a waveform diagram of the synchronization output signal 303 obtained when establishing the relationship shown in FIG. 49A. The input signal 301 to the comparator 107 assumes a waveform approximate to the sine waveform which varies, as shown in the Figure, with a fixed period.
On the other hand, in the output signal 303 of the comparator, in consequence of obtaining the reference voltage 302 as a threshold value, a difference between the input signal 301 and the reference voltage 302 appears, as can be understood from FIGS. 45A and 45B, in the form of a duty change in the output signal. If the recording/printing action is executed in synchronization with this output signal 303, the scatter in the density and a ruled-line deviation in an output image are caused. This results in a remarkable decline in terms of recording quality. For this reason, as already explained in relation to FIGS. 46A and 46B, the fluctuation in pitch P between the dots D can be seen, and consequently, the scatter in the density is produced as a result of recording. Particularly in the color printer, this may cause the deviation in the color registration.